Pigs catch swine flu the same way humans catch the flu: by breathing in virus-laden droplets from nearby infected animals. An infected pig coughs or sneezes, releasing influenza A virus into the air, and other pigs inhale it. The incubation period is short, typically one to three days, and an infected pig can shed the virus for seven to ten days, giving it plenty of time to spread through a herd.
Pig-to-Pig Spread
The primary transmission route is direct contact between pigs. Respiratory droplets and aerosols are the main vehicles, but the virus can also hitch a ride on contaminated objects like shared water troughs, feeding equipment, or pen surfaces. In commercial farming, where animals are housed in close quarters, a single infected pig can spark rapid spread through an entire barn.
The virus survives longer in cool, wet conditions. In water at around 4°C (roughly refrigerator temperature), swine influenza viruses remain infectious for 55 to 250 days. At body temperature (37°C), that drops to less than four days. On hard, nonporous surfaces at room temperature, the virus is typically inactivated within 24 hours. Neutral pH water (around 7.2) and low salinity favor survival, while acidic, alkaline, or salty environments destroy the virus faster. These details matter in practice because puddles, water lines, and damp barn floors can serve as temporary virus reservoirs in cooler months.
Humans Give It to Pigs, Too
One of the less obvious sources of swine flu in pig herds is the people who work with them. Human seasonal flu viruses have repeatedly jumped into pig populations around the world. Human H3N2 viruses have spilled over into pigs multiple times since 1968, with documented incursions in China, Europe, and the United States. The 2009 pandemic H1N1 virus continues to be transmitted from people back to pigs on a regular basis.
A well-documented case from France in 2018 illustrates how this works. A farm employee who was showing flu-like symptoms entered the breeding area. He showered and changed into dedicated clothing before entering, but he did not wear a mask or gloves. Within days, sows in the herd began showing respiratory illness. Genetic analysis confirmed the virus came from the employee, then spread pig to pig through the herd, and eventually infected a veterinarian who was treating the animals. This bidirectional cycle, human to pig and back again, is a key reason swine flu viruses keep evolving.
People typically shed influenza viruses for three to seven days, with peak shedding during the worst of their symptoms. That window is enough to introduce a new strain into a herd that has no immunity to it.
Why Pigs Are Uniquely Vulnerable
Pigs are sometimes called “mixing vessels” for influenza, and their biology explains why. The cells lining a pig’s respiratory tract carry two types of surface receptors that influenza viruses latch onto. One type is favored by human and swine flu viruses, and it’s found abundantly throughout the nose, trachea, bronchi, and lungs. The other type is preferred by bird flu viruses, and it’s concentrated deeper in the lungs, on the surfaces of the smallest airways and air sacs.
This dual receptor setup means pigs can be infected by flu viruses from multiple species. If a pig is simultaneously infected with a human virus and a bird virus, the two can swap genetic material inside the pig’s cells, producing a novel reassortant virus. Since 1998, the main swine flu strains circulating in U.S. pigs (H3N2, H1N2, and H1N1) have all contained genes originally from human influenza viruses.
How the Virus Enters a Farm
Even farms with biosecurity protocols have vulnerabilities. A risk assessment of pig farms in northern Italy identified several common entry points. In more than half the farms studied, there were no physical barriers like nets to prevent wild birds from accessing pig housing areas. About 23% of farms had personnel who also kept poultry in small flocks near the pig buildings for household egg production, creating a bridge for avian influenza strains to reach pigs.
Transport is another weak link. While more than 90% of the farms kept livestock transporters out of clean areas, nearly 40% did not provide company-issued or freshly laundered clothing to those transporters. Contaminated boots, coveralls, and equipment can carry the virus from one farm to the next. The virus also travels through ventilation systems. Poorly designed airflow that recirculates air within barns increases exposure for both pigs and workers.
Strains Currently Circulating in Pigs
Three main subtypes of influenza A circulate in pig populations worldwide: H1N1, H1N2, and H3N2. Surveillance at slaughterhouses in Vietnam during 2024 and early 2025 found swine influenza in about 11% of nasal swab samples tested. Of the positive samples, roughly 71% were H1N1 (the pandemic-origin strain), 25% were H3N2, and 4% were H1N2. All three subtypes were confirmed to be actively circulating, consistent with patterns seen in other major pork-producing regions.
These viruses are endemic in commercial pig populations globally, meaning they circulate year-round rather than appearing in seasonal waves the way human flu does. Outbreaks tend to spike when new animals are introduced to a herd, when weather shifts stress the animals, or when a novel strain enters through human or avian contact.
Vaccination and Herd Protection
Vaccination is the primary tool for controlling swine flu in commercial herds. Inactivated (killed-virus) vaccines matched to the circulating strain are effective at reducing illness and viral shedding. In controlled studies, pigs vaccinated with a strain-matched inactivated vaccine showed essentially no lung lesions after being exposed to the virus, and their nasal swabs tested negative for infectious virus. Shedding of viral genetic material in nasal secretions and oral fluids was also significantly reduced compared to unvaccinated pigs.
The catch is strain matching. Because swine flu viruses evolve and reassort frequently, a vaccine designed for one variant may offer incomplete protection against a different one. Farms often work with veterinarians to select vaccines based on the specific strains circulating in their region. Biosecurity measures, keeping wild birds out, enforcing clean clothing protocols for workers and transporters, and ensuring good ventilation, remain essential complements to vaccination.